Backport #2104 to 3-0-stable

Return empty when parsing a multi-part POST with only one end delimiter.

Fixed: #2103

Sending the following request in a browser generates a request with
with only one end delimiter.

```javascript
const formData = new FormData();
const request = new Request('http://127.0.0.1:8080/', {
  method: 'POST',
  body: formData,
});
const response = fetch(request);
```

```
curl 'http://127.0.0.1:8080/' \
  -H 'Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryR1LC4tR6ayskIXJm' \
  --data-raw $'------WebKitFormBoundaryR1LC4tR6ayskIXJm--\r\n'
```

This request is not compliant RFC7578, but is generated by major browsers such as
FireFox and Chrome.
Supporting this request will cause the multipart parser to return an empty value.

lib/rack/multipart/parser.rb

@@ -213,6 +213,7 @@ def initialize(boundary, tempfile, bufsize, query_parser)
 
         @sbuf = StringScanner.new("".dup)
         @body_regex = /(?:#{EOL}|\A)--#{Regexp.quote(boundary)}(?:#{EOL}|--)/m
+        @end_boundary_size = boundary.bytesize + 4 # (-- at start, -- at finish)
         @rx_max_size = boundary.bytesize + 6 # (\r\n-- at start, either \r\n or -- at finish)
         @head_regex = /(.*?#{EOL})#{EOL}/m
       end
@@ -279,7 +280,14 @@ def handle_fast_forward
             @state = :MIME_HEAD
             return
           when :END_BOUNDARY
-            # invalid multipart upload, but retry for opening boundary
+            # invalid multipart upload
+            if @sbuf.pos == @end_boundary_size && @sbuf.rest == EOL
+              # stop parsing a buffer if a buffer is only an end boundary.
+              @state = :DONE
+              return
+            end
+
+            # retry for opening boundary
           else
             # no boundary found, keep reading data
             return :want_read

test/spec_request.rb

@@ -1333,6 +1333,24 @@ def initialize(*)
     f[:tempfile].size.must_equal 76
   end
 
+  it "parse multipart delimiter-only boundary" do
+    input = <<EOF
+--AaB03x--\r
+EOF
+    mr = Rack::MockRequest.env_for(
+      "/",
+      "CONTENT_TYPE" => "multipart/form-data, boundary=AaB03x",
+      "CONTENT_LENGTH" => input.size,
+      :input => input
+    )
+
+    req = make_request mr
+    req.query_string.must_equal ""
+    req.GET.must_be :empty?
+    req.POST.must_be :empty?
+    req.params.must_equal({})
+  end
+
   it "MultipartPartLimitError when request has too many multipart file parts if limit set" do
     begin
       data = 10000.times.map { "--AaB03x\r\ncontent-type: text/plain\r\ncontent-disposition: attachment; name=#{SecureRandom.hex(10)}; filename=#{SecureRandom.hex(10)}\r\n\r\ncontents\r\n" }.join("\r\n")